1. Field of the Invention
The present invention relates to a frequency mixing apparatus. More particularly, the present invention relates to a frequency mixing apparatus for improving a voltage gain and linearity by separating a bias of a transconductor unit and a bias of a switching unit of the frequency mixer.
2. Description of the Related Art
A frequency mixer directly converts a Radio Frequency (RF) signal into a baseband data signal, or converts the RF signal into an Intermediate Frequency (IF) signal, and is widely used in a wireless communication transceiver.
FIG. 1 illustrates a conventional single-balanced frequency mixer.
Referring to FIG. 1, the single-balanced frequency mixer includes a transconductor 140 including single transistor MN1 112, a switching unit 130 including transistor MN2 108 and MN3 110, and an output unit 150 including resistors RL 100 and 102. When an input RF signal is applied to a gate 114 of MN1 112, the transconductor 140 converts the RF signal into a current signal Igm, and the converted current signal is input to common source node of transistors MN2 108 and MN3 110. At this point, Local Oscillation (LO) signals LO+ and LO−, via gate nodes 107 and 109, switch on/off transistors MN2 108 and MN3 110, so that a signal IF+ and IF− corresponding to a difference (intermediate frequency) between an RF frequency and an LO frequency is output via drains 104 and 106 of transistors MN2 108 and MN3 110. Generally, in a frequency mixer, when a current Igm flowing through a transistor of the transconductor unit 140 is increased, linearity improves. However, in a case of the single-balanced frequency mixer, when a current Igm is increased, currents flowing through the transistors MN2 108 and MN3 110 of the switching unit 130, and the resistors RL 100 and 102 of the output unit 150 also increase. When a current Isw flowing through the resistors RL 100 and 102 of the output unit 150 increases, a voltage drop over the resistor RL 100 or 102 increases, so that the range of fluctuation in an output voltage is reduced.
To address a limitation of the single-balanced frequency mixer, a frequency mixer which uses a current-bleeding technique has been proposed. The current-bleeding frequency mixer allows more current to flow through the transistor MN1 112 in the transconductor 140 and may reduce a current Isw flowing through the switching unit 130 and the load resistors RL 100 and 102 by using an additional current source. However, the additional current source operates as another noise source, and increases parasitic capacitance of a common source node of transistors MN2 108 and MN3 110 of the switching unit 130, so that a noise of the frequency mixer indirectly increases.
As described above, in the case of the single-balanced frequency mixer and the current-bleeding frequency mixer, bias currents of the transconductor units and the switching units are not independent. Instead, the bias currents of the transconductor units and the switching units are closely related to each other. Furthermore, since the sources of the transistors of the switching unit are connected to the drain of the transistor of the transconductor unit, it is difficult to optimize the biases of the transconductor unit and the switching unit. Generally, the linearity of the transconductor unit improves when a gate-source voltage and a drain-source voltage applied to the transistor of the transconductor unit are large. Therefore, the ability to improve the linearity of the transconductor according to a conventional frequency mixer structure is limited.